Process for preparing photographic emulsions



United States Patent Ofiice 3,341,333 Patented Sept. 12, 1967 3,341,333 PROCESS FOR PREPARING PHOTOGRAPHIC EMULSIONS Guenther H. Klinger and Peter A. Landskroener, Binghamton, N.Y., assignors to General Aniline 8; Film Corporation, New York, N.Y., a corporation of elaware No Drawing. Filed Oct. 18, 1963, Ser. No. 317,120 10 Claims. (Cl. 96-114) The present invention relates to a process for preparing photographic silver halide emulsions and has particular application to the washing of the emulsions after ripening to'remove uncombined metal salts without resortin to noodling.

In the conventional methods of preparing photographic silver halide emulsions, Par-t I is compounded with gelatin, water and the water-soluble alkali metal halides with or Without ammonia. Thereafter Part II, compounded with silver nitrate, is added to Part I while controlling the concentration, rate of addition and temperature so as to obtain the particular properties desired for the emulsion. The emulsion is then ripened, additional gelatin is added and the mixture is chilled and set to a gel. The gel must be broken up and Washed to remove the excess salts rior to melting and coating. These salts include those of silver which manifestly must be recovered for economical operations.

The breaking up and washing just mentioned are commonly carried out by first noodling the emulsion, that is, pressing it through a fine screen or collander which breaks it up into small strips or noodles. Then water is employed to wash out the salts from the slender gelatinosilver halide strips.

In recent years techniques have been developed which are applicable to some types of emulsions and which make it unnecessary, and some of the Washing steps hitherto required. Thus, it is possible to avoid the inconvenience, in many cases, that noodling involves. Two processes Which are widely employed for this purpose are, respectively, a coagulation wash technique and a precipitation wash technique. In the first of these, the gel is loosely coagulated and separated for washing. In the precipitation technique, the gel is precipitated in relatively large particles suitable for washing. As Will be pointed out below, both of these processes often have serious limitations. In particular, they cannot be used except with very dilute gelatin solutions. It would be desirable to be able to apply such techniques to more concentrated gelatino-silver halide emulsions and to accomplish this is one of the objects of this invention.

According to the present invention, therefore, certain important advantages of the precipitation wash and the coagulation wash techniques are obtained by a combination of processing steps which are very simply carried out. Moreover, photographic emulsions which previously could only be washed by the noodle wash technique can be satisfactorily washed without noodling and with high proportions of silver salt recovery. Further, the technique of the present invention not only results in excellent silver recovery but it also results in a considerable saving in labor, in time, and in other materials.

Although the advantages of the precipitation technique and the coagulation technique for washing gelatino-silver halide emulsions have been appreciated in the prior art where they were applicable, and the practice of noodling has been much reduced in recent years, it was generally assumed that these techniques could only be applied if the concentration of gelatin in the emulsion, at the beginning of the treatment, is kept very low and within predetermined limits, usually not more than 1%. In many cases, the use of these washing techniques has required that the in many cases, to go through the noodling gelatin concentration be less than about 0.3% concentration. If substantially larger proportions of gelatin are present, the precipitation technique, as previously carried out, will yield precipitates which swell up tremendously in volume as they are processed further. Such materials are very difficult to wash free of salts and the Washing must be considerably prolonged, becoming very inefiicient, especially if any satisfactorily high proportion of the unreacted silver salts are to be recovered.

The coagulation technique, which also has been used, ordinarily is not suitable for emulsions with gelatin con centrations substantially larger than 1%. Emulsions of such gelatin concentration, not substantially in excess of 1% by weight of gelatin, can be manipulated quite conveniently by the coagulation technique.

However, there are several disadvantages to the coagulation technique. In the first place, very large amounts of coagulating agents are necessary in order to precipitate the relatively large amounts of gelatin present in even moderately dilute emulsions. Thus, in the conventional preparation of emulsions, wherein Part I is put together with water, gelatin and the water-soluble halides, the large amount of gelatin required usually results in undesirable finished emulsion. For example, the viscosity of the finished emulsion often becomes so high that it interferes very seriously with the coating properties of the emulsion.

In connection with the usual precipitation Wash technique, the excessive swelling of the gelatin, when high percentages thereof are present, also militates against eifective and efiicient washing. Moreover, the percentage of silver salts recovered is relatively low and a great deal of economic loss is involved because of the failure to recover more of these. It is very desirable to recover at least 98% or more of the silver, although in prior art procedures, proportions of 10 to 15% and more, often have been lost.

It has now been found that by combining the coagulation technique with the precipitation technique, while using much smaller quantities of the coagulating and precipitating agents than are used to practice either technique per se, a synergistic effect is realized by which it i possible to wash silver halide emulsions with a high gelatin content, say up to 8% or higher, while avoiding the excessive swelling, low silver recovery and other disadvantages inherent in said techniques when practiced individually. Thus, the small amount of coagulating agent may be added after pH adjustment, the relatively small amount of precipitating agent, or vice versa. Strangely, the result obtained is the same regardless of the sequence in which the precipitating agent and coagulating agent are incorporated in the gel composition. Apparently it is the lesser amounts of both agents functioning as a unit which brings about the physical state in which the emulsion is most amenable to the Water wash rather than their order of addition.

The inventionmay accordingly be carried out by first adding to the silver halide emulsion relatively small amounts of conventional coagulating agents, for example 5% or less of the amount required for full coagulation. This procedure is applicable to emulsions which have fairly high proportions of gelatin therein. Thus, emulsions containing as much as 3, 4 or 5 even up to 8% of gelatin may be treated in this manner. By adding the small amount of the coagulating agent, that is, 15% or less of the amount required for full coagulation, and it is preferred to use even smaller amounts, i.e., below 5% of the normal, and then following this treatment by adding a suitable precipitating agent in an amount less than that required for full precipitation, absent the coagulating agent, i.e., 30% to 60% of such larger amount, a precipitate is obtained which has the following properties:

(1) The precipitate does not swell.

(2) It settles very rapidly.

(3) It washed efficiently.

(4) Excellent silver recovery is obtained, as high as 98% and more in most cases.

(5) Less precipitating medium such as the conventional electrolytes (alkali sulfates such as ammonium sulfate or sodium sulfate are preferred) is required for precipitation.

(6) This procedure permits the conversion of all noodle washed emulsions to precipitation wash.

To illustrate the invention further, the technique described above, in general terms, was applied in the production of several standard type batches indicated as DX for convenience. These batches were made up in relatively large quantities, for example, 100 to 160 kilograms or more per batch, and the procedure was found to be very satisfactory in all of them.

As specific examples, several are given below:

In all the emulsions exemplified below the same kind of gelatin was used and also the same proportions of gelatin, that is, constant gelatin concentration was maintained in all the samples at the beginning of the ripening period. Also the finished emulsion contained equal gelatin proportions.

More specifically, three emulsion batches were prepared. In each of these the concentration of gelatin at the beginning of the precipitation step was 2.9%. In the finished emulsion the gelatin proportions were 8.8% and in all cases the gelatin present in Part I, that is, the first part or component containing the initial portion of the total gelatin, was 60% of the final total. Hence, variables which might otherwise have been misleading in the results were eliminated.

EXAMPLE I An emulsion designated as DX-154 was prepared with a gel concentration of 2.9%. The pH of 210 liters of the emulsion liquid was adjusted by adding 50% sulfuric acid to bring it to 7 .2. A small amount of coagulating agent, namely, 100 grams of polystyrene sulfonate was added as a first treatment. The temperature was then lowered to 38 C. and the pH was adjusted by adding acetic acid to bring it to 4.2. Thereafter, 50 liters of a 50% aqueous solution of ammonium sulfate were added. A coarse precipitate resulted which settled rapidly and which was easily and readily washable. The settling time after each wash required only 10 minutes. For this particular product, the silver loss after washing was only 1.05 The viscosity was low, 12 cp., and the pAg was 8.66.

The following offers a comparison of the results obtained with our new procedure as against those obtained by conventional precipitation and conventional coagulation.

Conventional precipitation A batch called DX-113 was prepared by taking 100 liters of 50% ammonium sulfate solution and adding it to 210 liters of the emulsion liquid which contained 2.9% by weight of gelatin. It was subjected to the conventional precipitation technique. The precipitation temperature was 40 C. A fine precipitate was produced having a volume of 40 liters. On subsequent Washing the volume of this precipitate increased to 90 liters. After each washing a settling time of one hour was allowed. Washing was repeated a total of four times but it was difficult and inefiicient with this concentration of gel. The silver loss after 4 washes amounted to 13.5% of the silver incorporated in the original material. The viscosity in the finished product, after the four washings, was 8 cp. and the product had a pAg of 8.9.

Conventional coagulation An emulsion designated as DX-l46 was prepared with the same gel concentration as above, i.e., 2.9% by weight of gelatin. The pH was first adjusted to 7.2 by adding 50% sulfuric acid in sufficient quantity to obtain the desired pH value. Thereafter, 700 grams of polystyrene sulfonate were added as a coagulating agent. The temperature then was decreased by cooling the product to 38 C. and the pH was further adjusted to 4.2 by addition of acetic acid.

A fine precipitate resulted which settled very slowly. After each wash a settling time of about /2 hour was required. For 3 full washings the silver losses amounted to a total of 5% of original silver. The viscosity of the finished emulsion at 40 C. was excessively high, cp. This product had a pAg of 8.9.

EXAMPLE II This example was prepared in exactly the same manner as that of Example I, the procedure being repeated in detail and with particular care in washing. As in Example I a relatively small amount of polystyrene sulfonate was added to promote incipient coagulation and then the precipitating material, 50% aqueous solution of ammonium sulfate, was added. As in the previous example, a coarse precipitate resulted which settled rapidly and the settling time after each wash was only about 10 minutes. In this case, the silver loss after 4 washings was only 0.25%. The viscosity of the finished product, as in the case of Example I, was 12 cp. and the pAg was identical, 8.66.

EXAMPLE III The procedure was the same as in Example I except that the ammonium sulfate was added first, and the coagulating agent after the pH adjustment. The results were similar to those of Example I.

SUMMARY OF RESULTS OBTAINED ON SAMPLES DX-llB, 146 and 15 P Wash C Wash New Pro- Number of washes 4 3 4 pBr of finished emulsion Viscosity of finished emulsion, cp.

Silver loss in percent 0.25

The precipitating agent may be any inorganic electrolyte, preferably ammonium or sodium sulfate.

The coagulating agent which may be employed may be any organic polymeric material containing a watersolubilizing sulfonic acid or carboxyl acid group, such as polystyrene sulfonate, polyacrylic acid, polymethacrylic acid or an anionic surface-active agent such as dodecyl sulfate, kerylbenzene sulfonate (sulfonated condensation product of chlorinated kerosene with benzene), lauryl sulfonate, or the like (see U.S. Patents 2,527,261 and 2,527,263).

Obviouly, other variations may be made in the procedure and various known ingredients and treating materials, other than those named, may be added or used in the process. The basic advantage of the invention is that it makes possible to obtain unusually high silver recovery while still employing simple washing techniques and without requiring noodling of the emulsion. The washing steps, as applied to the coagulated precipitated materials, are very effective and are easily carried out. The procedure of the invention minimizes the labor and the losses which have been typical of prior techniques, especially when applied to emulsions containing substantially more than 1% of gelatin.

It is intended by the claims which follow to cover those variations and modifications which would suggest themselves to those skilled in the art, as far as the prior art permits.

What is claimed is:

1. The process for preparing gelatino-silver halide emulsions which comprises, in combination, the steps of adding in any sequence, after making, to such an emulsion containing in excess of 1% of gelatin, a small proportion of a coagulant namely not more than of the amount required to cause substantially complete coagulation of the emulsion, adding a precipitant in sufiicient proportions to obtain rapid settling of the gelatino-silver halide in a relatively coarse state of subdivision, and washing the precipitant to remove soluble salts and to recover a high proportion of unreacted silver.

2. The process for preparing gelatino-silver halide emulsions which comprises, in combination, the steps of adding, after making, to such an emulsion containing in excess of 1% of gelatin, a small proportion of a coagulant namely not more than 15% of the amount required to cause substantially complete coagulation of the emulsion, adding a precipitant in suflicient proportions namely, from 30% to 60% of the amount required for full precipitation in the absence of a coagulating agent, to obtain rapid settling of the gelatino-silver halide in a relatively coarse state of subdivision, and washing the precipitate to remove soluble salts and to recover a high proportion of unreacted silver salts.

3. The process as defined in claim 1 wherein the coagulant is selected from the class consisting of organic polymers containing a water-solubilizing group and anionic surface-active agents and the precipitating agent is an inorganic electrolyte.

4. A process as defined in claim 3 wherein the coagulating agent is a sulfonate of an aromatic polymer.

5. A process as defined in claim 4 wherein the aromatic polymer sulfonate is a polystyrene sulfonate,

6. A process as defined in claim 1 wherein the precipitating agent is selected from the group consisting of an alkali sulfate and ammonium sulfate.

7. A process as defined in claim 1 wherein the precipitating agent is sodium sulfate.

8. The process of preparing a gelatino-silver halide emulsion which comprises preparing an aqueous emulsion of gelatin and a silver halide, said emulsion containing at least 1% 'by weight of gelatin, adjusting the pH of said solution to make it acidic, adding a small proportion, not more than 15% of the quantity required for full coagulation, of a polystyrene sulfonate coagulating agent to cause incipient coagulation of the gelatin, thereafter adding substantial quantities of aqueous solution of ammonium sulfate namely, from 30% to of the amount required for full precipitation in the absence of a coagulating agent, to cause precipitation of a coarse precipitate which can readily be washed to remove unreacted salts, and Washing to remove unwanted salts.

9. A process as defined in claim 1 wherein the amount of coagulant is not more than about 18 grams per kilogram of gelatin in said emulsion.

10. A process as defined in claim 2 wherein the amount of coagulant is not more than about 18 grams per kilogram of gelatin in said emulsion and the amount of precipitant is from about 2.5 to about 5 kilograms per kilogram of gelatin in said emulsion.

References Cited FOREIGN PATENTS 1,279,110 11/1961 France.

884,840 12/1961 Great Britain.

NORMAN G. TORCHIN, Primary Examiner. R. H. SMITH, Assistant Examiner, 

1. THE PROCESS FOR PREPARING GELATINO-SILVER HALIDE EMULSIONS WHICH COMPRISES, IN COMBINATION, THE STEPS OF ADDING IN ANY SEQUENCE, AFTER MAKING, TO SUCH AN EMULSION CONTAINING IN EXCESS OF 1% OF GELATIN, A SMALL PROPORTION OF A COAGULANT NAMELY NOT MORE THAN 15% OF THE AMOUNT REQUIRED TO CAUSE SUBSTANTIALLY COMPLETE COAGULATION OF THE EMULSION, ADDING A PRECIPITANT IN SUFFICIENT PROPORTIONS TO OBTAIN RAPID SETTLING OF THE GELATINO-SILVER HALIDE IN A RELATIVELY COARSE STATE OF SUBDIVISION, AND WASHING THE PRECIPITANT TO REMOVE SOLUBLE SALTS AND TO RECOVER A HIGH PROPORTION OF UNREACTED SILVER. 